It has previously been proposed to deposit aluminum or aluminum alloy on a ferrous-metal substrate by dipping the latter into a molten bath of aluminum or aluminum alloy. An advantage of the aluminum-containing coating is that it generally has a higher resistance to corrosion than zinc (the most common galvanizing metal) and also has a greater resistance to high-temperature environments than coatings of lead, zinc or tin.
However, depositing a satisfactory coating of aluminum or an aluminum alloy onto a metal substrate is not simple since the coating character is affected by the oxidic surface normally present on the bath. Other problems which arise in the coating of aluminum onto metallic substrates from a bath or melt of the metal include the contamination of the workpiece surface and the high interfacial tension even when the substrate surface is clean and the aluminum is pure.
It has been proposed heretofore to overcome these disadvantages by introducing into a molten bath of aluminum, 0.005 to 0.13% by weight of sodium or potassium when the workpiece is a ferrous metal (iron or steel) or a metal such as chromium, nickel or titanium. Under these conditions, firmly adherent and uniform surface coating of aluminum alloys cannot be obtained.
Where ferrous metals constitute the substrate, the dipping of the workpiece into a bath of molten aluminum results in the formation of a reaction zone at which iron aluminum is formed. This metallic layer is brittle and of a thickness which depends upon the duration of immergence of the substrate in the aluminum bath.
The intermetallic iron-aluminide layer may have a thickness of 50 to 150 microns and is so brittle that, upon bending of the workpiece, the intermetallic layer cracks and results in spalling of the coating layer.
To avoid this disadvantage it has been proposed to provide barrier layers, e.g. of chromium, cobalt, molybdenum, nickel or tungsten, and to permit diffusion of the barrier metal into the structure of the ferrous metal underlying the barrier. Thereafter the aluminizing treatment is carried out with a diminished tendency toward formation of iron aluminide. However, even with this system it is not possible to completely prevent iron aluminide from forming and spalling, although to a lesser degree, thereby posing a problem. Furthermore, the diffusion operation and the deposition of the bearer layer is a time-consuming treatment which seldom is economical and may not be convenient or even possible.
It has also been proposed to control the thickness of the intermetallic (iron-aluminide) layer so that the latter has a thickness of 0.05 to 0.15 mm and contains 17 to 30% by weight aluminum by depositing the aluminum from a bath which contains magnesium and/or calcium as alloying elements. These systems are also not fully satisfactory. Finally, mention may be made of systems in which aluminum-silicon alloys are deposited upon metallic substrates from baths which contain calcium, strontium or barium to alter the structure of the layer. Even these systems have not proved fully satisfactory.